To be used together with @Configuration classes as follows,
enabling annotation-driven async processing for an entire Spring application context:

@Configuration
@EnableAsync
public class AppConfig {
}

MyAsyncBean is a user-defined type with one or more methods annotated with
either Spring's @Async annotation, the EJB 3.1 @javax.ejb.Asynchronous
annotation, or any custom annotation specified via the annotation() attribute.
The aspect is added transparently for any registered bean, for instance via this
configuration:

By default, Spring will be searching for an associated thread pool definition:
either a unique TaskExecutor bean in the context,
or an Executor bean named "taskExecutor" otherwise. If
neither of the two is resolvable, a SimpleAsyncTaskExecutor
will be used to process async method invocations. Besides, annotated methods having a
void return type cannot transmit any exception back to the caller. By default,
such uncaught exceptions are only logged.

NOTE: AsyncConfigurer configuration classes get initialized early
in the application context bootstrap. If you need any dependencies on other beans
there, make sure to declare them 'lazy' as far as possible in order to let them
go through other post-processors as well.

If only one item needs to be customized, null can be returned to
keep the default settings. Consider also extending from AsyncConfigurerSupport
when possible.

Note: In the above example the ThreadPoolTaskExecutor is not a fully managed
Spring bean. Add the @Bean annotation to the getAsyncExecutor() method
if you want a fully managed bean. In such circumstances it is no longer necessary to
manually call the executor.initialize() method as this will be invoked
automatically when the bean is initialized.

For reference, the example above can be compared to the following Spring XML
configuration:

The above XML-based and JavaConfig-based examples are equivalent except for the
setting of the thread name prefix of the Executor; this is because
the <task:executor> element does not expose such an attribute. This
demonstrates how the JavaConfig-based approach allows for maximum configurability
through direct access to actual componentry.

The mode() attribute controls how advice is applied: If the mode is
AdviceMode.PROXY (the default), then the other attributes control the behavior
of the proxying. Please note that proxy mode allows for interception of calls through
the proxy only; local calls within the same class cannot get intercepted that way.

Note that if the mode() is set to AdviceMode.ASPECTJ, then the
value of the proxyTargetClass() attribute will be ignored. Note also that in
this case the spring-aspects module JAR must be present on the classpath, with
compile-time weaving or load-time weaving applying the aspect to the affected classes.
There is no proxy involved in such a scenario; local calls will be intercepted as well.

Note that setting this attribute to true will affect all
Spring-managed beans requiring proxying, not just those marked with @Async.
For example, other beans marked with Spring's @Transactional annotation
will be upgraded to subclass proxying at the same time. This approach has no
negative impact in practice unless one is explicitly expecting one type of proxy
vs. another — for example, in tests.

Default:

false

mode

The default is AdviceMode.PROXY.
Please note that proxy mode allows for interception of calls through the proxy
only. Local calls within the same class cannot get intercepted that way; an
Async annotation on such a method within a local call will be ignored
since Spring's interceptor does not even kick in for such a runtime scenario.
For a more advanced mode of interception, consider switching this to
AdviceMode.ASPECTJ.